2023 03 14

2023-03-14

Java Monitor

• Busy Waiting

  object ThreadCommunication extends App {
    class SimpleContainer {
      private var value: Int = 0
      def isEmpty: Boolean = value == 0
      def set(newVal: Int) = value = newVal
      def get: Int = {
        val result = value
        value = 0
        result
      }
    }
  
    def naiveProdCons(): Unit = {
      val container = new SimpleContainer
  
      // Busy waiting => Computing Waste
      val consumer = new Thread(() => {
        println("[consumer] waiting")
        while(container.isEmpty) {
          println("[comsumer] actively waiting")
        }
        println("[consumer] I have consumed " + container.get)
      })
  
      val producer = new Thread(() => {
        println("[producer] computing...")
        Thread.sleep(500)
        val value = 42
        println("[producer] I have produced after long work, the value " + value)
        container.set(value)
      })
  
      consumer.start()
      producer.start()
    }
  }
  

• wait() & notify()

  def prodConsLargeBuffer(): Unit = {
      val buffer: mutable.Queue[Int] = new mutable.Queue[Int]
      val capacity = 3
  
      val consumer = new Thread(() => {
        val random = new Random()
  
        while (true) {
          buffer.synchronized {
            if (buffer.isEmpty) {
              println("[consumer] buffer empty, waiting...")
              buffer.wait()
            }
          }
          // there must be at least one value in the buffer
          val x = buffer.dequeue()
          println("[consumer] consumed " + x)
  
          // hey producer. there is empty space. Work!
          //        buffer.notify()
        }
  
        Thread.sleep(random.nextInt(500))
      })
  
      val producer = new Thread(() => {
        val random = new Random()
        var i = 0
  
        while (true) {
          buffer.synchronized {
            if (buffer.size == capacity) {
              println("[producer] buffer is full, waiting...")
              buffer.wait()
            }
  
            println("[producer] producing " + i)
            buffer.enqueue(i)
  
            // hey consumer! new food for you
            buffer.notify()
            i += 1
          }
          Thread.sleep(random.nextInt(500))
        }
      })
  
      consumer.start()
      producer.start()
  }
  
  prodConsLargeBuffer()